1. Field of the Invention
This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting knocking in an internal combustion engine.
2. Description of the Related Art
FIGS. 1 and 2 illustrate one example of an acceleration detector to which the present invention can be applied. The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an annular acceleration transducer assembly 3 disposed within the cavity 2. The housing 1 comprises a tubular, electrically conductive metallic bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring-shaped resinous outer case 7 bonded by a bonding agent 7a to the flange 6 of the bushing 4 so that the cavity 2 is defined therein.
The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an external output terminal 9 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 further includes an annular piezoelectric element 11 placed on the terminal plate 10, a washer-shaped output terminal 12 including a lead 12a connected to the external output terminal 9, an electrically insulating washer 13 disposed on the output terminal 12, an annular inertial weight 14 placed on the insulating washer 13 and a threaded ring-shaped stop nut 15 thread-engaged with the thread 4a on the tubular bushing 4.
As illustrated in FIG. 3, the output terminal 12 is made of a relatively thin metallic sheet and has a substantially ring-shaped portion 12b, a connection tab 12c substantially radially outwardly extending from the ring-shaped portion 12b and the lead 12a for an external connection. An electrically insulating tape or tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4 even when the output terminal 12 as well as the piezoelectric element 11 are eccentrically assembled.
As seen from FIGS. 4 to 7 inclusive, the ring-shaped stop nut 15 has an internal thread 15a in the inner circumferential surface thereof and a pair engagement grooves 15b formed in one of planar end surfaces 15c for receiving a tool 18 therein for rotating the stop nut 15 about the bushing 4 for thread engaging the stop nut 15 on the thread 4a of the bushing 4 as illustrated in FIG. 7. It is seen that the engagement grooves 15b are radially extending from the inner circumference to the outer circumference passing through the internal thread 15a of the stop nut 15.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exerted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on the internal combustion engine (not shown) by a suitable bolt (not shown) inserted into the central through hole 5 of the housing 1. The acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided from the output terminal 9 to be analyzed to determine as to whether or not a knocking signal which generates upon knocking of the internal combustion engine is involved. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
In the conventional acceleration detector as above described, since the engagement grooves 15b are formed in the planar end surface 15c of the stop nut 15 and extend through the internal thread 15a, burrs occur in the region at which the thread 15a and the engagement grooves 15b intersect. Such burrs interfere with the thread engagement between the stop nut 15 and the bushing 4, impeding an easy assembly of the acceleration transducer 3. If these burrs are to be removed, the internal thread 15a of the stop nut 15 must be additionally finished, increasing the number of manufacturing steps and cost.
Also, during the assembly of the acceleration detector, the tightening tool 18 is brought into engagement with the engagement grooves 15b and is rotated about its axis to tighten the stop nut 15 on the bushing 4 to securely hold the acceleration transducer 3. During this operation, the tightening tool 18 is often moved transversely back and forth as shown by an arrow 19 in FIG. 7 in order to properly insert the tool 18 into the engagement grooves 15b. This often causes the thread 4a on the bushing 4 to be damaged or destroyed by the tightening tool 18, which makes the tightening of the stop nut 15 on the bushing 4 difficult during the re-assembly of the detector.
Also, in the conventional acceleration detector as above described, during the tightening of the stop nut 15 on the bushing 4 to securely hold the acceleration transducer 3 in the detector housing 1, the output terminal 12 is also rotated due to the friction between the components in the acceleration transducer 3. This causes dislocation of the upwardly extending lead 12a relative to the external output terminal 9 to which the lead 12a must be connected. When the lead 12a is mechanically held in position by a suitable means such as when the external output terminal 9 and the lead 12a are connected, for example, the rotation of the ring portion 12b of the output terminal 12 causes a concentration of stress on transition areas A between the ring portion 12b and the connecting tab 12c, which may cause breakage at the transition area A.